Promises in Node.js 8.x Core

before-promisify.js

const fs = require('fs');

function exists(f, callback) {
    fs.stat(f, (err) => {
        callback(err ? false : true);
    });
}

function main() {
    const filename = './example.txt';
    const timestamp = new Date().toISOString();

    exists(filename, (fileExists) => {
        if (fileExists) {
            fs.appendFile(filename, `Updated file on ${timestamp}\n`, (err) => {
                if (err) {
                    console.err('Failed to update file');
                } else {
                    console.log('Successfully updated file');
                }
            });
        } else {
            fs.writeFile(filename, `Created file on ${timestamp}\n`, (err) => {
                if (err) {
                    console.err('Failed to create file');
                } else {
                    console.log('Successfully created file');
                }
            });
        }
    });
}

main();